A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive metal compound (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Being able to guarantee defect free imaging in extreme ultraviolet light (EUV) is an advantage for a successful introduction and acceptance in the market. Particles on the EUV reticles are one of the main sources of imaging defects. Because the EUV reticles are not covered by a membrane or pellicle (as common deep ultraviolet light (DUV) reticles have) which keeps contamination out of focus, they are prone to particle contamination, which may cause defects in a lithographic process. Cleaning and inspecting the reticle before moving the reticle to an exposure position is thus a desired aspect in a reticle handling process.
Current fast particle detection methods for DUV reticles and blanks use scattered light techniques. In this technique, a laser beam is focused on the reticle and a radiation beam that is scattered away from a specular reflection direction is inspected. In one embodiment, this is done by grazing incidence, although this is not strictly needed.
Particles on an object with a patterned surface, such as EUV reticles, will randomly scatter the light. By observing the illuminated surface with a microscope, the particles will light up as bright spots. The intensity of the spots is a measure of the size of the particle. However, these methods are not easily transferable to objects having non-flat surfaces such as an EUV reticle, because the patterned surface structure of the EUV reticle will contribute to the scattered light.